Heat pipe with control



.June 23, 1970 J. T. KEISER 3,516,487

HEAT PIPE WITH CONTROL Filed Feb. 21, 1968 JOHN /fE/SER,

United States Patent O 3,516,487 HEAT PIPE WITH CONTROL John T. Keser,King of Prussia, Pa., assignor to General Electric Company, acorporation of New York Filed Feb. 21, 1968, Ser. No. 707,201 Int. Cl.G05d 23/00 U.S. Cl. 16S-105 1 Claim ABSTRACT 0F THE DISCLOSURE Heat isconveyed from a nominally constant-output heat source to a heatutilization device by a heat pipe employing a transfer medium whosevapor pressure varies rapidly with temperature change. In order tomaintain the operating temperature approximately constant with changesin consumption by the utilization device (or in the event of variationin the supposedly constant source output) an inert gas reservoir isconnected to the end of the heat pipe remote from the source. Thereservoir tempera'ture is maintained approximately constant in order tokeep the pressure of the inert gas approximately constant by placing itin close thermal communication with the heat pipe whose temperature ittends to regulate.

REFERENCES Structures of Very High Thermal Conductance, G. M. Grover, T.P. Cotter, G. F. Erickson, Journal of Applied Physics, vol. 35, No. 6,June 1964, pp. l990l991, The Heat Pipe, K. Thomas Feldman, Jr., Glen H.Whiting, Mechanical Engineering, February 1967, pp. 30-33.

SPECIFICATION This invention pertains to devices for the transfer ofheat by the ow of a vaporized condensable fluid.

Certain heat sources, such as isotope heat generators, produce an outputof heat power which varies very slowly with time as compared withvariations in'power requirements, is not adjusted to match currentrequirements but is left effectively free-running, and consequently ismost conveniently utilized by a suitable thermal conductor capable ofremoving the heat so produced at a temperature convenient forutilization and compatible with maintaining the physical integrity ofthe source. A known device suitable for this purpose is the heat pipe,which s described in the cited references (which are included herein byreference), and consists usually of a closed structure containing a heattransfer material which is capable of existence as a liquid at thedesired temperature, and has at that temperature a vapor pressure whichis substantial but low enough to be withstood by the container. Theliquid is vaporized at the heat source and the resultant vapor flows byits own pressure to the cooler portions of the structure, where it iscondensed, yielding its latent heat. This procedure would ordinarilysimply result in transfer of all the liquid from the hot to the coolerportions of the structure, and the process would stop. However, thewalls of the structure are covered with a porous wick structure which iswet by the condensed vapor, now liquid, and conveys the liquid back tothe hot region, thus establishing a continuous circulation. This processis most simply practiced in the absence of any gas other than the vaporof the liquid. However, in order to operate at moderate pressures anddesirably high temperatures, it is often necessary to employliquids`which it is difficult to produce and maintain free of othergases. For example, a common heat pipe is one in which the structure isa tube of stainless steel lined with a wick of stainless steel mesh andthe uid is molten sodium metal. Sodium readily forms hydrides, so thatto obtain sodium vapor free of traces of hydrogen is very Ficedifficult. However, a moderate amount of foreign nonreactive gas is notvery harmful, since the continued flow of sodium vapor from the hot tothe cool region tends to drive the foreign gas to the end of the pipeaway from the heat source. Its pressure is, of course, that of thevapor.

It is apparent that the operating temperature of the heat pipe will bewhatever is required to produce a sufficient flow of vapor to transmitthe heat being passed through the pipe. If a constant heat input isprovided, and the thermal impedance to removal of that amount of heatfrom the cool part of the pipe is increased, the operating temperatureof the pipe will increase until the temperature at the cool part issufficient to cause flow of the constant amount of heat through theincreased impedance. This may be undesirable. It has been taught in theprior art to restrict such temperature increase by limiting the increasein vapor pressure in the heat pipe by providing a reservoir of foreigngas connected to an addenum to the cool end of the pipe. Under normaloperating conditions, the addenum is occupied chiefly by the foreigngas, and remains comparatively cool. If, however, the removal of heat atthe cool end of the pipe becomes insuflicient at the normal operatingtemperature to remove all the heat being put in at the hot end, theforeign gas is compressed only slightly (because of the comparativelylarge volume of the reservoir) and the vapor begins to condense in theaddenum, from which the excess heat may flow into the surroundingwithout a marked increase in the pressure and temperature of the vapor.This device, while commendable, has the defect that the pressure in thegas reservoir is markedly a function of temperature, and may vary widelywith the ambient temperature. My invention comprises placing the gasreservoir in good thermal contact with the most nearly constanttemperature part of the system.

This has two advantages, when applied to a nominally constant outputheat source. So long as the discarding of heat from the addendum of theheat pipe at the desired operating temperature is suicient to maintainthe sourceI at the desired operating temperature, the gas pressure inthe reservoir will remain approximately constant. If, however, use ofthe full area of the addendum is insufficient (with the removal at theregular cool part) to discard all the heat being produced, so that thetemperature of the source begins to rise, the pressure in the gasreservoir will rise also, permitting the heat pipe to operate at asomewhat higher temperature and so to increase the heat flow at thenormal cool part and also at the addendum. This may be achieved byplacing the foreign gas reservoir either directly in good thermalcontact with the heat source itself (which may be inconvenient from adesign standpoint) or, which analysis shows to be slightly superior, ingood direct thermal contact with the portion of the heat pipeimmediately adjacent to the heat source, and thus still, albeitindirectly, in good thermal contact with the heat source.

Thus I achieve the desirable result of stabilizing the operatingtemperature of the heat pipe system for normal variations in operatingconditions and at the same time of making the system somewhatself-stabilizing under abnormal conditions outside of the normal rangeof variations.

For the better explanation and understanding of my invention, I haveprovided figures of drawing n which FIG. 1 represents schematically asystem according to my invention in which the foreign gas reservoir islocated in thermal contact with the heat source itself, and

FIG. 2 represents schematically a system according t0 my invention inwhich the foreign gas reservoir is located in thermal contact with apart of the heat pipe immediately adjacent to the heat source.

Referring to FIG. 1, there is represented a heat source 12 from whichthere extends a heat pipe 14, which has an insulated portion 16extending immediately from heat source 12 to a condensing section 18which is in contact with a utilization device 20, which accepts heatfrom part 18 and rejects it, at a lower temperature, to a radiator 22.Beyond condensing section 18 lies addendum 24 of the heat pipe which isprovided with a radiator 26; and the end of addendum 24 is connected bya tube 28 to foreign gas reservoir 30, which is represented in goodthermal contact with heat source 12.

FIG. 2 is identical with FIG. 1, except that foreign gas reservoir 30 isrepresented as being in good thermal contact with insulated portion 16of heat pipe 14, and thus also in good thermal contact with heat source12.

In a particular embodiment of my invention, preferable for manypurposes, the heat pipe is a tube of 0.65 inch inside diameter and wallthickness of .0465 inch, charged with 60 grams of metallic sodium linedwith wicking of stainless steel. The foreign gas may be argon at apressure of 51.6 torr, the volume of the foreign gas reservoir beingapproximately 30 cu. in. This is for operation at a nominal temperatureof 650 degrees C.

The heat source 12 may be an isotope heat source; and the utilizationdevice 20 may bea thermoelectric pile or any other suitable heatconverter.

In normal operation, utilization device 20 absorbs heat from part 18 ofthe heat pipe 14, converts it to electrical energy, and rejects the heatat reduced temperature through radiator 22. Any excess of heat isdissipated along a part of addendum 24 and thence by radiator 26, theremainder of addendum 24 being lled with gas from reservoir 30 at apressure determined by the temperature of that reservoir, which will bedetermined by the temperature of the heat source 12 for the embodimentof FIG. 1, or by the nearly identical temperature of insulated portion16 of heat pipe 14.

lf abnormal conditions should cause the vapor to ll the entire addendum24 without dissipating all the output of source 12 at the desiredtemperature, the temperature of heat source 12 will rise and increasethe ternperature of foreign gas reservoir 30, either by direct contactor through section 16 of pipe 14, and the vapor pressure and thetemperature in heat pipe 14 will increase, increasing the dissipation ofheat at least at addendum 24, tending to stabilize the system even underthe ablnormal condition.

The mass of inert gas required is determined by the usual gas lawPV=MRT, where P is the vapor pressure of the working substance at thedesired operating temperature T, V is the volume of the system less thevolume normally occupied by the vaporized Working substance, M is themass of gas required, R is the usual gas constant.

While the lack of simple analytic expressions for various thermalfunctions renders a simple general quantitative analysis of theoperation of my invention difcult, it is possible to give a simpleformula for determining the volume of reservoir required to provide agiven temperature variation for a given change in operating conditions.It should be observed that, While the gas in the reservoir is maintainedat substantially the heat pipe operating temperature, the gas in thepipe itself, since it serves to push back the vapors which convey heat,will be at a somewhat lower temperature. It is usually a sufficientlygood approximation to determine for one set of operating conditions theratio of the absolute temperature of the gas in the pipe to the absolutetemperature of the gas in the reservoir, and to assume that this ratio KVolume of reservoir P(II) vol. pipe (II) P(I) vol. pipe (I) K temp. res.(II) K temp. res. (I)

P(I) P(II) Temp. res. (I) Temp. res. (II) where the abbreviations havethe following meanings:

P(I) and P(II) are, respectively, pressure at rst and :second sets ofconditions Vol. pipe (I) and vol. pipe (II) are the volume of inert gasin the heat pipe under each of the indicated sets of operatingconditions (I) or (II) Temp. res.` (I) and temp. res. (II) are theabsolute temperature of the reservoir under each of the indicated setsof operating conditions (I) or (II).

K has been defined in the preceding.

All pressures must be in the same units; all temperatures must be in thesame units; all volumes must be inthe same units; but it will beobserved that the dimensions of the right-hand side of the equation areof the form Pressure x Volume/Temperature-ePressurc/Tcmperature so thatthe pressure and temperature units cancel out. Thus there is no need forconsistency among the different kinds of units; pressure in pounds persquare inch, temperature in absolute Reaumur, and volumes in cubicmillimeters will still yield the required volume in cubic millimeters.

What is claimed is: 1. In a heat pipe system comprising: a heat source;a heat pipe in thermal contact with the heat source,

comprising:

a condensing section in thermal contact with a utilization device anaddendum for dissipating heat not absorbed by the said utilizationdevice; a reservoir of foreign gas connected to the terminal end of thesaid addendum; the improvement wherein said reservoir of foreign gas isin good thermal contact with the said heat source.

References Cited UNITED STATES PATENTS 1/1966 Grover 165-105 OTHERREFERENCES MEYER PERLIN, Primary Examiner A. W. DAVIS, AssistantExaminer U.S. Cl. X.R.

